! The prevalence of food allergy has experienced an unprecedented increase in Western societies, rising by as much as 20% in a recent ten-year period. We have previously described a role for mucosa- associated commensal bacteria in protection from allergic sensitization in mice. To understand how the microbiota regulates allergic disease in humans, we colonized germ free mice with human bacteria from the feces of healthy or cow?s milk allergic (CMA) infants. Our preliminary data shows that colonization with a healthy human microbiota is sufficient to protect mice against sensitization to the cow?s milk allergen b-lactoglobulin (BLG), whereas colonization with a human CMA microbiota fails to protect. By analyzing operational taxonomic units (OTUs) differentially abundant between our human fecal donors (4 healthy, 4 CMA) we have defined a microbiota signature that distinguishes the CMA and heathy populations in both the human donors and the colonized mice, emphasizing the clinical relevance of our gnotobiotic model. RNAseq analysis of gene expression in ileal intestinal epithelial cells (IECs) across these 8 donors revealed differentially expressed genes (DEGs) that separate healthy- and CMA- colonized mice and also highly correlate with the abundance of ileal OTUs related to the allergic phenotype in the CMA-colonized mice. In the experiments proposed we will examine the hypothesis that allergic sensitization to food is influenced by dysbiosis at the level of the epithelium. We will further refine our OTU signature and examine whether the CMA infant microbiome is an atopic microbiome in Aim 1. Aim 2 will explore the mechanism(s) by which the CMA microbiota drives an allergic response by identifying the downstream immunological consequences of the induced ileal IEC DEGs. We will phenotypically and functionally characterize populations of dendritic cells, regulatory T cells, innate lymphoid cells and T effector cells in the lamina propria of sensitized healthy- and CMA-colonized mice. The robust, pre-clinical gnotobiotic model we describe will provide an ideal system in which to identify key host-microbial interactions that contribute to allergic sensitization to food. !